Add EPUB version of lessons (build with pandoc)

**This is a work in progress. Don't merge it.**

This add a entry in `Makefile` to build EPUB(2) from `_site/book.html`:

~~~
$ make epub
~~~

**Problem**: `pandoc` will not include HTML `class`es from `_site/book.html` into EPUB (more information at jgm/pandoc#1052).